Recently, instead of conventional CRT (Cathode Ray Tube), lots of flat panel display devices such as a liquid crystal display device (LCD), PDP (Plasma Display Panel), and an OLED (Organic Emitting Diode) have been developed.
Among these, the liquid crystal display device comprises a thin film transistor substrate, a color filter substrate, and a liquid crystal display panel having liquid crystal injected between the two substrates.
The liquid crystal is a non-light-emitting material which controls the amount of the light incident from the outside and displays it on a screen, rather than a light emitting material which itself emits light. Thus, generally the liquid crystal display device is provided with a back light unit for illuminating light on the liquid crystal display panel.
The back light unit comprises a light source portion for generating light, and an optical sheet which is disposed under the liquid crystal display panel and changes the characteristics of the light emitted from the light source portion. The back light unit is classified into two types of units, i.e., an edge type of a unit in which the light source portion is provided on the side of the light guide plate, and a direct type of a unit in which the light source portion is provided under the liquid crystal display panel, according to the positions of the light source portion.
The optical sheet comprises a diffusion sheet for diffusing the light from the light source portion and supplying it to the liquid crystal display panel, and a prism sheet for vertically condensing the light diffused by the diffusion sheet to a plane of the liquid crystal display panel.
Herein, since the diffusion sheet generally used is isotropic in view of the light scattering property, it has a problem that it reduces the luminance in a certain direction to an extent more than needed.
In order to solve the problem, JP Patent Application Publication No. 2001-159704 discloses an anisotropic light scattering film comprising particles dispersed in the resin for providing high anisotropicity to the incident light. From this, the anisotropicity of the light scattering becomes higher, whereby reduction in the luminance in a certain direction can be suppressed, and a uniform luminance can be provided.
However, these conventional anisotropic light scattering films use expensive raw materials for preparation of an anisotropic light scattering film, which leads to high production cost, and difficult processes such as orientation. Thus, the preparation is not easy, which gives a problem that the productivity is deteriorated.
Therefore, in the preparation of the anisotropic diffusion sheet, it is preferable to reduce the production cost relative to the conventional sheets, and eliminate the complicated and difficult processes such as orientation to establish a simpler production process, thus the productivity being improved.
Further, in the case where the anisotropic diffusion sheet is used in a liquid crystal display device, if a prism sheet is eliminated from the liquid crystal display device by performing both of the light diffusion function and the light condensing function, a back light unit having a slim appearance and a liquid crystal display device comprising the same are preferably provided.